This adds the basic building blocks for generating changelogs from a
list of commits. The output is Markdown, and each release section is
generated using a minimal, custom template language.

In an early iteration we used Liquid, but this was found to suffer from
unresolved security issues. Alternative template languages had their own
share of issues that made using them not an option. The template engine
introduced in this commit is the bare minimum that we need for changelog
generation, and works by compiling the user provided template to an ERB
template; making sure the user can't run arbitrary code. Using ERB
allows us to offload the heavy lifting of a template engine to ERB,
instead of having to write and maintain such an engine ourselves.

The use of a template engine gives users control over how they want to
present their changelog data, without GitLab having to provide
potentially dozens of settings to enable this.

This builds on the changes introduced in merge requests
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/49243 and
https://gitlab.com/gitlab-org/gitaly/-/merge_requests/2842.

See https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/1369 for
more information.